C-reactive protein binds to Fc gamma RI in transfected COS cells

C-Reactive protein (CRP) is an acute phase serum protein in man that binds to certain bacterial polysaccharides and to components exposed on damaged cells. CRP is bound by receptors on phagocytic cells and functions as an opsonin for its ligands. Interactions of CRP with a specific CRP receptor (CRP-R) and with the high affinity receptor for IgG, Fc gamma RI, on monocytic cells have previously been demonstrated. It was not possible to fully characterize CRP binding to Fc gamma RI in these studies, since cells and cell lines expressing Fc gamma RI also have the CRP-R. In the present study we examined the interaction of CRP with Fc gamma RI in COS-7 cells transfected with a cDNA encoding this receptor. Expression of Fc gamma RI and specific CRP binding to transfected cells were demonstrated by flow cytometry. By two-color analysis, the cell population binding CRP was the same as the population that bound the Fc gamma RI-specific mAb 10.1 and 32.2 CRP inhibited the binding of radiolabeled IgG1 and IgG4 by up to 60%. A CRP molecule that was mutated in the amino acid sequence homologous to the IgG sequence proposed to interact with Fc gamma RI failed to bind to transfected cells, but retained the ability to bind to the CRP-R on monocytic cells. These studies confirm the binding of CRP to Fc gamma RI and identify a site on CRP that is essential for this binding.